A conventional power supply circuit for driving a light emitting element, as illustrated in FIG. 4, is comprised of a step-up circuit 1, a step-up voltage control circuit 15, a voltage regulator 11, a CPU 13, an AF unit 12 and an AF light emitting diode 17 for emitting light in response to the operation of a light emitting diode driving transistor 16.
The step-up circuit 1 includes a coil 2, a rectifier diode 3, a switching transistor 4 and a step-up capacitor C.sub.1. One terminal of the coil 2 is connected to the (+) terminal of a battery VB, and the (-) terminal of the battery VB is connected to a ground point. The other terminal of the coil 2 is connected to the anode of a rectifier diode 3, and the cathode of the rectifier diode is connected to a pin P.sub.1. The other end of the coil 2 is also connected to the collector of a switching transistor 4. The emitter of the transistor is connected to the ground point, and the base of the switching transistor 4 is connected to a pin P.sub.2. One electrode of a step-up capacitor C.sub.1 is connected to the pin P.sub.1, and e other electrode thereof is connected to the ground point.
An input terminal of the voltage regulator circuit 11 is connected to the pin P.sub.1, while the output thereof provides a stabilized power supply voltage VDD. This voltage is applied to one electrode of a capacitor C.sub.2. The other electrode of the capacitor C.sub.2 is connected to the ground point. One electrode of a light emitting capacitor C.sub.3 is connected to a junction point between resistors R.sub.4 and R.sub.3, and the other electrode of the capacitor C.sub.3 is connected to the ground point. The other end of the resistor R.sub.4 is connected to the pin P.sub.1 of the step-up circuit 1 and the other end of the resistor R.sub.3 is connected to the anode of the light emitting diode 17. The cathode of the AF light emitting diode 17 is connected to the ground point via the collector and emitter path of the light emitting diode driving transistor 16.
The step-up circuit 1 of the above described power supply circuit for driving the light emitting element employs known circuitry. The step-up capability of this circuit is determined by losses of the parts thereof and duty cycles of step-up clock signals applied to the switching transistor 4. The step-up voltage control circuit 15, which is comprised of a comparator or the like, performs the function of maintaining a constant step-up voltage by stopping the step-up operation when the step-up voltage increases over a predetermined voltage, and reducing the duty of the step-up clocks applied to a pin P.sub.6 from the CPU 13 In this arrangement, the stepped up voltage is applied to the input of the voltage regulator 11. The regulated voltage VDD of approximately 5 V is applied from the output of the regulator to the CPU 13 and the AF unit 12. The light emitting diode driving transistor 16 for driving the AF light emitting diode 17 is turned ON/OFF in response to signals output from a pin P.sub.5 of the AF unit 12. The resistor R.sub.3 and the light emitting capacitor C.sub.3 serve as a power supply circuit for the AF light emitting diode 17. The AF light emitting diode 17 emits light with the energy from electric charges with which the light emitting capacitor C.sub.3 is charged. The electric charges with which the light emitting capacitor C.sub.3 is charged is supplied from the step-up capacitor C.sub.1. If a large charge current flows in the light emitting capacitor C.sub.3 in a charging cycle, the predetermined voltage is not applied to the input side of the voltage regulator 11, with the result that the prescribed regulated voltage VDD can not be maintained. The resistor R.sub.4 is provided in order to avoid this problem. A surge current flowing in the light emitting capacitor C.sub. 3 is restrained by resistor R.sub.4.
As described above, the conventional power supply circuit for driving the light emitting element requires a capacitor having a large capacity. A capacitor with a large capacity typically employs an aluminum electrolytic capacitor. If such a capacitor is packaged into a small-sized camera, the problem arises that the required space is not sufficient and the costs of the elements are increased.
Medium and large-sized capacitors, such as aluminum electrolytic capacitors, etc. are assembled manually, resulting in the disadvantage that the assembly costs are increased.